1. Field of the Invention
The present invention relates to a laser machining apparatus, which applies a laser beam at an angle that is set to be inclined with respect to the surface of a workpiece, in order to form oblique holes in the surface of the workpiece.
2. Description of the Related Art
Aircraft engines, e.g., gas turbine engines, burn fuel in a combustor by injecting the fuel into the combustor while at the same time sending combustion air, which is compressed by a rotary compressor, into the combustor. During burning of the fuel, the combustor produces a high-temperature combustion gas under high pressure, and delivers the combustion gas in order to rotate a centrifugal or axial-flow gas turbine.
Since the combustor burns fuel and generates high-temperature combustion gas, the combustor is exposed to a considerably high-temperature environment. The gas turbine has turbine blades, which also are exposed to the high-temperature environment. U.S. Patent Application Publication 2008/0223835 A1 (hereinafter referred to as “Prior Art Document 1”) and U.S. Patent Application Publication 2012/0051941 A1 (hereinafter referred to as “Prior Art Document 2”) disclose cooling holes formed obliquely in the surface of the combustor for introducing secondary air into the combustor, so as to bring about a cooling effect that lowers the temperature of the combustor surface.
According to Prior Art Document 1 and Prior Art Document 2, a laser beam is applied to a machining surface of the combustor in order to form oblique holes, which function as cooling holes. When the laser beam is applied to the machining surface of the combustor, the laser beam is reflected by the machining surface, which tends to cause sputtering. Consequently, molten metal scum (hereinafter referred to as “dross”) frequently is produced by the laser beam and becomes deposited on a machining surface that is machined subsequently. Such dross is liable to lower the accuracy with which the laser beam machines the machining surface, and to increase the period of time required to machine the machining surface.
Occasionally, a practice is carried out to set the angle at which the laser beam is applied to the machining surface to a considerably small value, to thereby elongate the oblique holes that serve as cooling holes. If the angle at which the laser beam is applied to the machining surface is of a small value, then the wall thickness of the workpiece, which is machined by the laser beam, increases, thereby increasing the period of time required to machine the workpiece with the laser beam and resulting in a tendency to produce even more dross.
According to one solution, as disclosed in U.S. Patent Application Publication 2009/0151996 A1 (hereinafter referred to as “Prior Art Document 3”), a sacrificial layer is provided on the surface of a workpiece, so as to control the tapered shape of a hole to be formed in the workpiece, and to prevent burrs from being formed as well as to prevent sputtered particles from becoming deposited on the workpiece.